Decades of U.S. dominance kept stability in the South China Sea. Now, China is making waves to claim its 'blue national soil.'

War, as the dark quip goes, is God's way of teaching us geography. This month's lesson, thanks to Vladimir Putin, is Ukraine and the Crimean peninsula. Next, thanks to China's increasing assertiveness, could be Southeast Asia—especially the South China Sea.

In "Asia's Cauldron," prolific traveler and strategist Robert D. Kaplan notes that the South China Sea is "becoming the most contested body of water in the world." Roughly bounded by Indonesia to the south, Singapore, Malaysia and Vietnam to the...